An Air Data System (“ADS”) provides sensed telemetry informing pilots, navigators or Vehicle Management System computers of air parameter(s) affecting aircraft stability. These air parameters include, for example, air speed, air temperature and air pressure, each being useful for navigation and flight control. The ADS exists in many forms, for example, as mechanical, opto-mechanical or opto-electronic devices.
One mechanical ADS includes a Pitot tube, which affixes to the exterior of the aircraft. The Pitot tube is a pneumatic measuring instrument that receives air external to the aircraft and determines air speed based upon air pressure. A typical Pitot tube uses a pressure transducer to measure the pressure of air received through a plurality of holes. Determined air pressure is combined with air density measurements and air temperature measurements using the well-known Bernoulli equation to calculate air speed. Pilots and Vehicle Management Systems use this information for decisions regarding aircraft stability, flight control and navigation.
The prior art ADS has limitations. With respect to the Pitot tube, for example, if aircraft velocity is too low, the pressure transducer may lack requisite sensitivity to provide meaningful telemetry data, such that transducer signal noise or error may be greater than meaningful differences in pressure measurement. Alternatively, if the velocity is very high (e.g., supersonic), certain assumptions, such as those regarding incompressibility of air flow in Bernoulli's equation, are violated and the measurement is incorrect. Accordingly, Bernoulli's equation must be altered to compensate for various breached assumptions. This compensation process must continue in rapidly changing conditions of altitude and air density. It is, therefore, desirable to improve accuracy and operability of the ADS.
One improvement is an Optical Air Data System (“OADS”), which uses light to determine parameters of air speed. The OADS transmits light pulses into the atmosphere and receives light that is reflected, or “backscattered”, from aerosols towards the aircraft. Aerosols are fine solids and/or liquid particles suspended in air or other gases.
The OADS may also measure the Doppler effect, receiving the backscattered light and measuring return frequency to determine speed. However, the prior art OADS relies on scattered light that is unpredictable because of varying aerosol distributions. For example, aerosol distribution varies significantly with altitude and cloud content. In addition, some regions of the atmosphere contain too few aerosols to enable reliable air data measurements. Thus, aerosol-based optical air data systems cannot determine the air speed at all altitudes frequented by modern aircraft. Finally, an OADS that relies solely on aerosol scattering cannot determine air temperature or air pressure. Air temperature and pressure are critical air data parameters for determining air density and Mach number. Hence, there is a need to improve optical air data systems and methods.